An exact determining of the depth of occurrence of chemically aggressive beds is necessitated by the fact that such beds as, say, saliferous ones prove to be unstable and tend to dissolve upon getting in contact with the drilling mud which results in a badly affected quality of the latter and in formation of cavities in the well being drilled, whereby some amount of the drilling mud is lost due to penetration into the cavities thus formed. All this imposes extra expenditures concerned with restoration of the drilling mud characteristics and replenishing the amount thereof.
One of the measures taken to eliminate such troubles resides in covering the aggressive beds with a casing string. If, however, the depth of occurrence and the zone of a bed are detected insufficiently accurately the casing string might be run down deeper than it is required or, conversely, it might prove shorter than necessary and thus fail to cover the whole bed.
Known in the art at the present time are some electrometric methods of detecting the depth of occurrence of the rock beds by measuring the potentials of intrinsic polarization of rock in the well with the use of an electrode sunk into the well through the cable. The electrode is then forced against the wall of the well and the electromotive force between the electrode sunk in the well and another electrode on the daylight surface is measured (cf. a text-book "Studies into sections of oil and gas wells by the intrinsic potential method" by B. Yu. Vendenshtain, issued 1966 by Nedra Publishers, Moscow/in Russian/). Said method involves much time to be spent for running the electrode down the well and hoisting it therefrom, as well as a great scope of jobs concerned with the determination of the depth of the bed occurrence.
Another method of detecting the depth of occurrence of a water-developing bed by transiently ceasing the mud injection into the well, followed by resuming the mud circulation, whereupon an amount of the drilling mud diluted with the bed fluid and returned from the well is registered and a change in the mud density as compared to the initial one is estimated. Besides, in the course of such a determination there is measured the period of time from the beginning of mud injection to the return of an amount of drilling mud diluted with the bed fluid (cf.n e.g., USSR Inventor's Certificate No. 484,301 cl. E 21 B, 47/04 published in the bulletin "Discoveries, inventions, industrial designs and trademarks" No. 34, 1975 /in Russian/).
Said method, however, is inapplicable for detecting the depth of occurrence of a chemically aggressive bed as such a bed does not change the density of the drilling mud upon getting in contact therewith.